Processing method for group IBIIIAVIA semiconductor layer growth

ABSTRACT

A method of forming a doped Group IBIIIAVIA absorber layer for solar cells by reacting a partially reacted precursor layer with a dopant structure. The precursor layer including Group IB, Group IIIA and Group VIA materials such as Cu, Ga, In and Se are deposited on a base and partially reacted. After the dopant structure is formed on the partially reacted precursor layer, the dopant structure and partially reacted precursor layer is fully reacted. The dopant structure includes a dopant material such as Na.

CLAIM OF PRIORITY

This application claims the benefit of U.S. Provisional Application Ser.No. 60/826,521 filed Sep. 21, 2006 entitled “Processing Method andApparatus for Group IBIIIAVIA Semiconductor Layer Groups,” which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to method and apparatus for preparing thinfilms of doped semiconductors for photovoltaic applications.

BACKGROUND

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon, whichis in the form of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce cost ofsolar cells for terrestrial use. One way of reducing the cost of solarcells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se,Te, Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1-x)Ga_(x) (S_(y)Se_(1-y))_(k),where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employedin solar cell structures that yielded conversion efficienciesapproaching 20%. Among the family of compounds, best efficiencies havebeen obtained for those containing both Ga and In, with a Ga amount inthe 15-25%. Absorbers containing Group IIIA element Al and/or Group VIAelement Te also showed promise. Therefore, in summary, compoundscontaining: i) Cu from Group IB, ii) at least one of In, Ga, and Al fromGroup IIIA, and iii) at least one of S, Se, and Te from Group VIA, areof great interest for solar cell applications.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1. The device 10 is fabricated on a substrate 11, such as a sheetof glass, a sheet of metal, an insulating foil or web, or a conductivefoil or web. The absorber film 12, which comprises a material in thefamily of Cu(In,Ga,Al)(S,Se,Te)₂, is grown over a conductive layer 13 ora contact layer, which is previously deposited on the substrate 11 andwhich acts as the electrical ohmic contact to the device. The stack ofthe substrate 11 and the conductive layer 13 forms a base 20. The mostcommonly used contact layer or conductive layer in the solar cellstructure of FIG. 1 is Molybdenum (Mo). If the substrate itself is aproperly selected conductive material such as a Mo foil, it is possiblenot to use a conductive layer 13, since the substrate 11 may then beused as the ohmic contact to the device. The conductive layer 13 mayalso act as a diffusion barrier in case the metallic foil is reactive.For example, metallic foils comprising materials such as Al, Ti, Ni, Cumay be used as substrates provided a barrier such as a Mo layer isdeposited on them protecting them from Se or S vapors. The barrier isoften deposited on both sides of the foil to protect it well. After theabsorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO orCdS/ZnO stack is formed on the absorber film. Radiation 15 enters thedevice through the transparent layer 14. Metallic grids (not shown) mayalso be deposited over the transparent layer 14 to reduce the effectiveseries resistance of the device. The preferred electrical type of theabsorber film 12 is p-type, and the preferred electrical type of thetransparent layer 14 is n-type. However, an n-type absorber and a p-typewindow layer can also be utilized.

The preferred device structure of FIG. 1 is called a “substrate-type”structure. A “superstrate-type” structure can also be constructed bydepositing a transparent conductive layer on a transparent superstratesuch as glass or transparent polymeric foil, and then depositing theCu(In,Ga,Al)(S,Se,Te)₂ absorber film, and finally forming an ohmiccontact to the device by a conductive layer. In this superstratestructure light enters the device from the transparent superstrate side.A variety of materials, deposited by a variety of methods, can be usedto provide the various layers of the device shown in FIG. 1. It shouldbe noted that although the chemical formula of copper indium galliumsulfo-selenide is often written as Cu(In,Ga)(S,Se)₂, a more accurateformula for the compound is Cu(In,Ga)(S,Se)_(k), where k is typicallyclose to 2 but may not be exactly 2. For simplicity the value of k willbe assumed as 2. It should be further noted that the notation “Cu(X,Y)”in the chemical formula means all chemical compositions of X and Y from(X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means allcompositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)₂ means thewhole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to1, and Se/(Se+S) molar ratio varying from 0 to 1.

The first technique that yielded high-quality Cu(In,Ga)Se₂ films forsolar cell fabrication was co-evaporation of Cu, In, Ga and Se onto aheated substrate in a vacuum chamber. This is an approach with lowmaterials utilization and high cost of equipment.

Another technique for growing Cu(In,Ga)(S,Se)₂ type compound thin filmsfor solar cell applications is a two-stage process where metalliccomponents of the Cu(In,Ga)(S,Se)₂ material are first deposited onto asubstrate, and then reacted with S and/or Se in a high temperatureannealing process. For example, for CuInSe₂ growth, thin layers of Cuand In are first deposited on a substrate and then this stackedprecursor layer is reacted with Se at elevated temperature. If thereaction atmosphere also contains sulfur, then a CuIn(S,Se)₂ layer canbe grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Gastacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)₂absorber.

Sputtering and evaporation techniques have been used in prior artapproaches to deposit the layers containing the Group IB and Group IIIAcomponents of the precursor stacks. In the case of CuInSe₂ growth, forexample, Cu and In layers were sequentially sputter-deposited on asubstrate and then the stacked film was heated in the presence of gascontaining Se at elevated temperature for times typically longer thanabout 30 minutes, as described in U.S. Pat. No. 4,798,660. More recentlyU.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositinga stacked precursor film comprising a Cu—Ga alloy layer(s) and an Inlayer to form a Cu—Ga/In stack on a metallic back electrode layer andthen reacting this precursor stack film with one of Se and S to form theabsorber layer. U.S. Pat. No. 6,092,669 described sputtering-basedequipment for producing such absorber layers.

One prior art method described in U.S. Pat. No. 4,581,108 utilizes a lowcost electrodeposition approach for metallic precursor preparation. Inthis method a Cu layer is first electrodeposited on a substrate coveredwith Mo. This is then followed by electrodeposition of an In layer andheating of the deposited Cu/In stack in a reactive atmosphere containingSe to obtain CIS.

Prior investigations on possible dopants for Group IBIIIAVIA compoundlayers have shown that alkali metals, such as Na, K, and Li, affect thestructural and electrical properties of such layers. Especially,inclusion of Na in CIGS layers was shown to be beneficial for increasingthe conversion efficiencies of solar cells fabricated on such layersprovided that its concentration is well controlled. Inclusion of Na intoCIGS layers was achieved by various ways. For example, Na was diffusedinto the CIGS layer from the substrate if the CIGS film was grown on aMo contact layer deposited on a Na-containing soda-lime glass substrate.This approach, however, is hard to control and causes non-uniformitiesin the CIGS layers depending on how much Na diffuses from the substratethrough the Mo contact layer. Therefore the amount of Na doping is astrong function of the nature of the Mo layer such as its grain size,crystalline structure, chemical composition, thickness, etc. In anotherapproach (see for example, U.S. Pat. No. 5,626,688), a diffusion barrieris deposited on the soda-lime glass substrate to stop possible Nadiffusion from the substrate into the absorber layer. A Mo contact filmis then deposited on the diffusion barrier. An interfacial layercomprising Na is formed on the Mo surface. The CIGS film is then grownover the Na containing interfacial layer.

During the growth period, Na from the interfacial layer gets includedinto the CIGS layer and dopes it. The most commonly used interfaciallayer material is NaF, which is evaporated on the Mo surface before thedeposition of the CIGS layer by the co-evaporation technique (see, forexample, Granath et al., Solar Energy Materials and Solar Cells, vol:60, p: 279 (2000)). U.S. Pat. No. 7,018,858 describes a method offabricating a layer of CIGS wherein an alkali layer is formed on theback electrode (typically Mo) by dipping the back electrode in anaqueous solution containing alkali metals, drying the layer, forming aprecursor layer on the alkali layer and heat treating the precursor in aselenium atmosphere. Another method of supplying Na to the growing CIGSlayer is depositing a Na-doped Mo layer on the substrate, following thisby deposition of an un-doped Mo layer and growing the CIGS film over theundoped Mo layer. In this case Na from the Na-doped Mo layer diffusesthrough the undoped Mo layer and enters the CIGS film during hightemperature growth.

It should be noted that the methods described above include the alkalimetal early in the process and the alkali metal, such as Na,participates in the overall reaction as Cu, In, Ga and Se, optionally Sreact with each other forming the CIGS(S) compound film on the base. Forexample, a stack may be formed in accordance with prior art approacheswhere a Na-containing layer may first be deposited on a base comprisinga Mo layer coated on a substrate. The Na-containing layer may bedeposited on the Mo surface and then this may be followed by thedeposition of a metallic precursor comprising Cu, In and Ga. When thisstack is heated up in presence of Se to form a CIGS layer, all elements,i.e. Cu, In, Ga, Se and Na participate in this reaction. Same is truewhen Cu, In, Ga and Se are co-evaporated on a heated substrate surfacecovered by a Mo layer and a Na-containing layer. In these approaches,participation of the alkali, such as Na, in the reaction, may yieldNa-containing phases such as Na-selenide (and/or Na-sulfide if S ispresent) compounds as alloys between Na and any one of Cu, In and Ga.The Na amount that is actually useful for doping, then needs to beexperimentally determined since some Na is used for other reactions.Formation of excess phases such as Na—Se and or S may also reduceavailability of Se and/or S to the actual CIGS(S) compound formation andthus cause problems with repeatability of the process as well asmaterials utilization. Sodium also influences the morphology and grainsize of CIGS-type materials. Rudmann et al. (Thin Solid Films, vol:431-432, p: 37, 2003), for example, observed a reduction in grain sizeof the CIGS films when Na was available during the growth of thecompound layers.

As the review above demonstrates, controlled doping of Group IBIIIAVIAcompound layers with alkali metals improve their quality in terms ofyielding higher efficiency solar cell devices, however, there is stillneed for alternative methods to introduce the dopants into the compoundlayers in an efficient manner that does not negatively impact theirmorphological and electrical properties.

SUMMARY OF THE INVENTION

The present invention provides a process to introduce one or more dopantmaterials into absorbers used for manufacturing solar cells. In a firststage of the process, a precursor stack is prepared and partiallyreacted to have non-metallic phases in it. In a second stage, a dopantlayer including a dopant material is formed on the partially reactedprecursor. In a third stage, the partially reacted precursor layer isfully reacted in presence of the dopant material from the dopant layerto form an absorber layer.

In an aspect of the present invention, a multilayer structure to formabsorber layers for solar cells is provided. The multilayer structureincludes a base comprising a substrate layer, a partially reactedprecursor layer formed on the base and a dopant layer on the partiallyreacted precursor layer. The dopant layer of the structure includes aGroup IA material. The partially reacted precursor layer of thestructure includes at least one of a non-metallic phase of GroupIB-Group VIA material and a non-metallic phase of Group IIIA-Group VIAmaterial. Accordingly, the Group IB material is Cu, Group IIIB materialis at least one of In and Ga, Group VIA material is at least one of Seand S, and Group IA material includes one of Na, K and Li.

In another aspect of the present invention, a process of forming a dopedGroup IBIIIAVIA absorber layer on a base is provided. The processincludes: depositing at least one Group IB and Group IIIA materials andVIA material on the base, forming a partially reacted precursor layer bypartially reacting these materials, depositing a dopant-bearing film onthis partially reacted precursor layer, and forming a doped absorberlayer by fully reacting the partially reacted precursor layer inpresence of dopant material from the dopant-bearing film. Here, thepartial reaction of the at least one Group IB and Group IIIA materialswith at least one Group VIA material results in a precursor layer havingat least 50% non-metallic phase which includes at least one of anon-metallic phase of Group IB-Group VIA material and a non-metallicphase of Group IIIA-Group VIA material. The dopant-bearing film includesa dopant material having at least one of Na, K and Li.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a solar cell employing a GroupIBIIIAVIA absorber layer;

FIG. 2A is a schematic illustration of a pre-absorber structure of thepresent invention including a dopant layer formed on a partially reactedprecursor layer;

FIG. 2B is a schematic illustration of an absorber layer formed afterreacting the pre-absorber structure shown in FIG. 2A;

FIG. 3 shows a process flow chart in accordance with one embodiment ofthe present invention; and

FIG. 4 is a schematic illustration of a solar cell manufactured using anembodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a process to introduce one or more dopantmaterials into a precursor layer to manufacture absorber layers forsolar cells. The process of the present invention generally includesthree stages. In a first stage of the inventive process a primarystructure such as a partially reacted precursor layer is initiallyprepared. The partially reacted precursor layer may be formed as a stackincluding layers of materials. In a second stage of the presentinvention, a secondary structure or a dopant layer, a dopant materialwith or without another material is formed on the partially reactedprecursor layer. The primary and secondary structures together form apre-absorber structure or pre-absorber stack. And, in a third stage,annealing of the pre-absorber structure forms a doped absorber layer or,in the art as often referred to as, a doped compound layer.

As shown in FIG. 2A, in one embodiment, a multilayer stack 100 of thepresent invention includes a pre-absorber structure 102 formed on a base104 including a substrate 106 and a contact layer 108. The pre-absorberstructure 102 includes a partially reacted precursor layer 110 and adopant-bearing film 112 which is formed on top of the partially reactedprecursor layer 110. The dopant-bearing film 112 may be 2-100 nm thick,preferably 5-20 nm thick. In this embodiment, the partially reactedprecursor layer 110 may be formed using a two-step process. During afirst step, at least one Group IB and at least one Group IIIA materialmay be deposited on the base to form a metallic layer. For example, Cu,In and Ga metals are deposited on the base to form a metallic layercomprising Cu, In and Ga on the contact layer. The amounts of Cu, In andGa metals may be adjusted to a desired composition. The desiredcomposition may have a Cu/(In+Ga) molar ratio of 0.8-1.0 and aGa/(In+Ga) molar ratio of 0.1-0.3. Various techniques such aselectrodeposition, PVD, CVD, nano-particle deposition, ink depositionetc. may be employed to form the metallic layer. In the second step ofthe process, the partially reacted precursor layer 110 is formed on thedopant-free base 104 by reacting, preferably partially reacting, themetallic layer with at least one Group VIA material. Partial reactionmeans that the precursor layer is not converted to pure Group IBIIIAVIAcompound such as a CIS, CGS or CIGS compound. For example, for CIGSgrowth, the precursor layer after the second stage does not contain onlyCIGS but may comprise several phases such as CIS, CGS, CIGS, Cu, In, Ga,Cu—In alloys or intermetallics, Cu—Ga alloys or intermetallics, Cu—Sephase, (In,Ga)Se phases, Cu—In—Ga phases, etc. For the purpose of thisinvention, out of these various phases, CIS, CGS, CIGS, Cu—Se, and(In,Ga)Se phases are considered non-metallic phases, whereas, Cu, In,Ga, Cu—In alloys, Cu—Ga alloys and Cu—In—Ga alloys are consideredmetallic phases.

The reaction of the metallic layer with the Group VIA material(s) may beachieved by various means. For example, a Group VIA material such as Semay be deposited on the metallic layer forming a stack and then thestack may be heated up in a furnace, in a Rapid Thermal Processing unitor laser annealing unit etc. Alternately the metallic layer may beexposed to heat and at the same time to a vapor (such as Se vapor) orgas (such as hydrogen selenide) comprising a Group VIA material. Duringthe reaction, the temperature may be in the range of 250-550° C. and thereaction time may be in the range of 1-60 minutes. The phase compositionof the partially reacted precursor layer after the second step of theprocess may be: (i) at least one of CuIn-selenide/sulfide,CuGa-selenide/sulfide and CuInGa-selenide/sulfide phases, and at leastone of Cu-selenide/sulfide, In-selenide/sulfide, Ga-selenide/sulfide andInGa-selenide/sulfide phases, or, (ii) at least one of the Cu, In, Ga,Cu—In alloy, Cu—Ga alloy, Ga—In alloy and Cu—In—Ga alloy metallic phasesand at least one of the Cu-selenide/sulfide, In-selenide/sulfide,Ga-selenide/sulfide, InGa-selenide/sulfide, CuIn-selenide/sulfide,CuGa-selenide/sulfide, CuInGa-selenide/sulfide non-metallic phases, or,(iii) at least two of the Cu-selenide/sulfide, In-selenide/sulfide,Ga-selenide/sulfide, and InGa-selenide/sulfide non-metallic phases.

If metallic and non-metallic phases are both present in the partiallyreacted precursor layer (such as case (ii) above), it is preferred thatthe non-metallic phases constitute more than 50% (molar), morepreferably more than 80% of the chemical composition of the partiallyreacted precursor layer. It is important to have the partially reactedprecursor layer to contain selenide and/or sulfide phases with wellformed micro-structure.

Once it is formed on the base, at least one dopant-bearing film 112 isthen provided on the partially reacted precursor layer 110 to completethe pre-absorber structure 102. The dopant material may be a Group IAmaterial such as Na and the means of providing may comprise depositing aNa-bearing film on the surface of the partially reacted precursor layer110. Depositing Na may be achieved by several techniques such as PVD,CVD, solution deposition, ink coating, dipping the partially reactedprecursor layer into a Na-containing solution and drying etc. TheNa-bearing film may be any film that has Na in its composition, such asNa-sulfide, selenide, chloride, fluoride etc.

As shown in FIG. 2B, once completed, the multilayer stack 100 is heatedup, optionally in presence of additional Group VIA material species totransform the pre-absorber stack 102 into an absorber layer 120comprising a doped Group IBIIIAVIA layer with large grain structure.During this reaction stage, the multilayer stack 100 may be annealed ata temperature range of 400-600° C. for a period of time of about 5-60minutes, preferably 10-30 minutes. The heat initiates a reaction betweenconstituents of the stack including the dopant and the partially reactedprecursor layer. During this stage a pure Group IBIIIAVIA compound layeris formed and at the same time doped by the dopant. Preferably,additional Group VIA material(s) is supplied to the partially reactedprecursor layer during the reaction step.

In the above example, the substrate may be a flexible metallic substratesuch as a steel web substrate having a thickness of about 25-125micrometers, preferably 50-75 micrometers. Similarly, the contact layer(Mo, W, Ru, Os or Ir, etc.) may be 200-1000 nm thick, preferably 300-500nm thick. The above given precursor layers or stacks may have athickness in the range of 400-1000 nm, preferably, 500-700 nm. The flowchart shown in FIG. 3 further exemplifies the process flow of thepresent invention described above.

FIG. 4 shows a solar cell 200 formed by further processing of the abovedescribed absorber layer 120 shown in FIG. 2B. Solar cells may befabricated on the absorber layers of the present invention usingmaterials and methods well known in the field. For example a thin CdSlayer 202 (<0.1 microns) may be deposited on the surface of the absorberlayer 120 using the chemical dip method. A transparent window 204 of ZnOmay be deposited over the CdS layer using MOCVD or sputteringtechniques. A metallic finger pattern (not shown) is optionallydeposited over the ZnO to complete the solar cell.

The invention will now be described through some examples utilizingelectrodeposited Cu, In, Ga layers and Na as the dopant. It should benoted that the invention may be practiced using various other techniquesfor film depositions including approaches such as PVD, CVD, particledeposition, ink coating, electroless deposition etc. and various otherdopants such as K, Li, Bi, P.

EXAMPLE 1 Na-Doped CIGS Film Formation

Cu, In and Ga layers are electrodeposited on a base forming a metalliclayer. The base may comprise a substrate such as a foil or sheetsubstrate coated with a contact layer. In this case the base issubstantially free of the dopant Na or barrier layers are providedwithin the base to avoid Na diffusion from the base into the CIGS layerbeing formed. The contact layer may comprise conductors such as Mo, W,Ta, Ti, Cr, Ru, Ir, Os etc., preferably Ru. Cu, In and Ga may be in theform of mixtures, alloys or discrete stacks with various structures suchas Cu/Ga/In, Cu/In/Ga, Cu/Ga/Cu/In, Cu/In/Cu/Ga, etc. stacks. During thefirst reaction step of the process, Se and heat are provided to themetallic layer to initiate a reaction between Cu, In, Ga and Se.Selenium may be provided through a vapor source, such as Se vapor or agas such as H₂Se, or Se solid may be deposited on the metallic layer byvarious methods such as PVD, electrodeposition, ink coating etc. Thetemperature may be in the range of 250-550° C. and the reaction time maybe in the range of 1-60 minutes, longer times being utilized for lowerreaction temperatures. The goal of the first reaction step is to providepartial reaction between the species of Cu, In, Ga and Se in absence ofNa so that a partially reacted precursor layer with a first compositionis formed with a first crystalline structure without interference fromthe dopant. This first composition may comprise at least one of thebinary selenides of Cu, In and Ga and possibly a ternary phase of CISand/or CGS and/or CIGS.

After the first reaction step a Na-containing layer is deposited on thepartially reacted precursor layer with the first composition usingvarious methods such as PVD, spraying, dipping, ink writing etc. ThisNa-containing layer may be a Na-fluoride, chloride, bromide, acetate,sulfide, selenide layer or any other layer that contains Na. The Naamount in the Na-containing layer may correspond to an amount that is0.01-1% (atomic) of the final CIGS layer that is formed after the secondreaction step which will be described. A second reaction step is thencarried out where the partially reacted species of the precursor layerwith the fist composition further react among themselves and possiblywith externally supplied Se species in presence of Na provided by theNa-containing layer to form a CIGS layer that is doped with Na. If theSe amount provided during the first reaction step is adequate to formthe stoichiometric CIGS layer there is no need to provide additional Seduring the second reaction step. However, if this is not the case, it ispreferable to provide additional Se to the growing layer during thesecond reaction step. It should be noted that although Na in thisexample is introduce through deposition of a Na-containing layer on theprecursor layer with the first composition before the second reactionstep, it is possible to introduce Na into the film during the secondreaction step. In this case Na or Na-containing species may be providedfrom vapor phase during the second reaction step. The temperature duringthe second reaction step may be in the range of 400-600° C. and thereaction time may be in the range of 5-60 minutes, shorter reactiontimes corresponding to higher reaction temperatures.

EXAMPLE 2 Na-Doped CIGSS Film Formation

Cu, In and Ga layers are electrodeposited on a base forming a metalliclayer. The base may comprise a substrate such as a foil or sheetsubstrate coated with a contact layer. In this case the base issubstantially free of the dopant Na or barrier layers are providedwithin the base to avoid Na diffusion from the base into the CIGS layerbeing formed. The contact layer may comprise conductors such as Mo, W,Ta, Ti, Cr, Ru, Ir, Os etc., preferably Ru. Copper, In and Ga may be inthe form of mixtures, alloys or discrete stacks with various structuressuch as Cu/Ga/In, Cu/In/Ga, Cu/Ga/Cu/In, Cu/In/Cu/Ga, etc. stacks.During the first reaction step of the process, Se and heat are providedto the metallic layer to initiate a reaction between Cu, In, Ga and Se.Selenium may be provided through a vapor source, such as Se vapor or agas such as H₂Se, or Se solid may be deposited on the metallic layer byvarious methods such as PVD, electrodeposition, ink coating etc. Thetemperature may be in the range of 250-550° C. and the reaction time maybe in the range of 1-60 minutes, longer times being utilized for lowerreaction temperatures. The goal of the first reaction step is to providepartial reaction between the species of Cu, In, Ga and Se in absence ofNa so that a partially reacted precursor layer with a first compositionis formed with a first crystalline structure without interference fromthe dopant. This first composition may comprise at least one of thebinary selenides of Cu, In and Ga and possibly a ternary phase of CISand/or CGS and/or CIGS.

After the first reaction step a Na-containing layer is deposited on thepartially reacted precursor layer using various methods such as PVD,spraying, dipping, ink writing etc. This Na-containing layer may be aNa-fluoride, chloride, bromide, acetate, sulfide, selenide layer or anyother layer that contains Na. The Na amount in the Na-containing layermay correspond to an amount that is 0.01-1% (atomic) of the final CIGSSlayer that is formed after the second reaction step. A second reactionstep is then carried out where the partially reacted species of thepartially reacted precursor layer with the first composition furtherreact among themselves and with externally supplied S species inpresence of Na to form a CIGSS layer that is doped with Na. Depending onthe Se amount provided during the first reaction step the S and Secontent in the final CIGSS layer may be controlled. It should be notedthat although Na in this example is introduce through deposition of aNa-containing layer on the partially reacted precursor layer with thefirst composition before the second reaction step, it is possible tointroduce Na into the film during the second reaction step. In this caseNa or Na-containing species may be provided from vapor phase during thesecond reaction step. The temperature during the second reaction stepmay be in the range of 400-600° C. and the reaction time may be in therange of 5-60 minutes, shorter reaction times corresponding to higherreaction temperatures. Although Se is introduced into the metallic layerduring the first reaction step in this example, it is also possible thatS is introduced during the first reaction step forming a Cu—In—Ga—Sprecursor layer with the first composition and then Se is introducedduring the second reaction step to form the doped CIGSS layer. It isalso possible that S and Se are introduced together during the first andsecond reaction steps.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. A process of forming a doped Group IBIIIAVIA absorber layer on abase, comprising: depositing at least one Group IB and Group IIIA andVIA material on the base; forming a partially reacted precursor layer bypartially reacting the at least one Group IB and Group IIIA materialswith at least one Group VIA material, wherein partially reacting the atleast one Group IB and Group IIIA materials with at least one Group VIAmaterial results in the partially reacted precursor layer having atleast 50% non-metallic phase; depositing a dopant-bearing film on thepartially reacted precursor layer, the dopant-bearing film comprising adopant material including at least one of Na, K and Li; and fullyreacting the partially reacted precursor layer with the dopant materialfrom the dopant-bearing film to form the doped Group IBIIIAVIA absorberlayer.
 2. The process of claim 1, wherein the Group IB material is Cu,Group IIIA materials are In and Ga, and at least one Group VIA materialcomprises Se.
 3. The process of claim 2 further comprising supplying agaseous environment containing Se during the step of fully reacting. 4.The process of claim 2 further comprising supplying a gaseousenvironment containing S during the step of fully reacting.
 5. Theprocess of claim 2 further comprising supplying a gaseous environmentcontaining S during the step of partially reacting.
 6. The process ofclaim 2 further comprising supplying a gaseous environment containing Sand Se during the step of fully reacting.
 7. The process of claim 1,wherein the step of partially reacting comprises annealing at atemperature range of 250-550° C. for about 1-60 minutes.
 8. The processof claim 1, wherein the step of fully reacting comprises annealing at atemperature range of 400-600° C. for about 5-60 minutes.
 9. The processof claim 1, wherein the at least one Group IB, Group IIIA and Group VIAmaterial comprise Cu, In, Ga and Se elements.
 10. The process of claim1, wherein the step of depositing the at least one Group IB, Group IIIA,and Group VIA material on the base comprises electroplating.
 11. Theprocess of claim 1, wherein the step of depositing the dopant-bearingfilm comprises dip coating the dopant material.
 12. The process of claim1, wherein the step of depositing the dopant-bearing film comprisesvapor depositing the dopant material.
 13. The process of claim 1,wherein the step of partially reacting the at least one Group IB andGroup IIIA materials with at least one Group VIA material results in thepartially reacted precursor layer having at least 80% non-metallicphase.
 14. The process of claim 1, wherein the non-metallic phasecomprises at least one of selenides and sulfides of Cu, In, Ga, CuIn,CuGa, InGa, and CuInGa.
 15. The process of claim 13, wherein thenon-metallic phase comprises at least one of selenides and sulfides ofCu, In, Ga, CuIn, CuGa, InGa, and CuInGa.